(1) Field of the Invention
The present invention relates to antennas, and more particularly to antennas which can be used in broadcast and mobile communication radio stations and also in such locality as elongate underground shopping areas and tunnels.
(2) Description of the Related Art
Antennas of the kind to which the present invention relates have recently been finding applications familiar to our life, such as radio broadcast, television broadcast and mobile communication systems such as pocket bells, portable telephones and car telephones.
Radio base station facilities to be installed in tunnels, passageways, rooms, underground shopping areas, etc., are provided in such a way that they are not obstructive to traffics of passers-by. Specifically, main bodies are installed inside ceiling surface boards, while antennas are mounted thereon such that they penetrate the ceiling surface boards toward passages.
Antennas used are non-directivity antennas of rod or bar-like or planar type. Bar-like antennas are frequently used as base station antennas, because they are elongate in form and less noticeable from a scenic stand point.
FIGS. 1A and 1B show, in a perspective view and a side view respectively, a prior art antenna. Referring to these Figures, reference numeral 11 designates an antenna element, 12 a metal conducting plate, and 13 an antenna element supporting member. This antenna is shown in Japanese Patent Application Kokai Publication No. Hei 8-51314.
Many mobile stations have small output power, because they are required from their characters to be small in size and light in weight and consume low power. On the base station side, on the other hand, the transmission output power is set to be relatively high because of the necessity for the base station to cover pertinent large areas for such purposes as calling mobile stations.
Also, in order to maintain a good communication condition with many mobile stations, antennas are installed at high levels such as building roofs. Their working frequency ranges from several hundred MHz to several GHz. As for the radio frequency to be used, higher frequencies are more advantageous from the standpoint of the effective frequency utilization and the antenna size of the mobile station.
With the above prior art antenna, the necessary number of base station antennas is increased when it is intended to cover zones extending along elongate service areas such as roads. When the overall radiation power is increased, the electromagnetic wave energy radiated in the transversal direction of the road is wasted.